Smoke detector including porous housing means



Feb. 24, 1970 R. LENEMARK :TAL 3,497,303

I SMOKE vnm gcfwoa ncwmns, POROUS nousnm MEANS Filed July 11, 1967 United States Patent 3,497,303 SMOKE DETECTOR INCLUDING POROUS HOUSING MEANS Robert B. Enemark, Weymouth, and Donald F. Steele, 'Cohasset, Mass., assignors to Electro Signal Lab, Inc., Weymouth, Mass., a corporation of Massachusetts Filed July 11, 1967, Ser. No. 652,447 Int. Cl. G01n 21/00 U.S. Cl. 356-103 7 Claims ABSTRACT OF THE DISCLOSURE A smoke detector withan opaque block housing a light source and a photodetector in respective passages through which light paths intersect in a space surrounded by the block and walls formed of a light-obstructive, smoketransmissive porous material.

In one known type of detector of smoke or like fluidborne particles, light from an internal source is projected along a path into an enclosed, otherwise dark space. A photodetector views along a second path intersecting the first and responds only if smoke particles are present in the intersection and scatter light, part of which is scattered along the second path to the photodetector. In such smoke detectors there are conflicting requirements. Air passages to the dark space must be sufliciently open to admit a detectable concentration of smoke. On the other hand, light from external sources or from surfaces bounding the dark space must be kept low in relation to the fraction of light scattered to the photodetector by the available concentration of smoke particles.

One object of the present invention is to provide an arrangement of a local light source and photodetector which minimizes unwanted reflections from the dark space to the photodetector. A further object is to increase the volume of air passages to the dark space while minimizing admission of external light, thereby to increase the ratio of smoke-scattered light to unwanted light.

A still further object is to prevent smoke and other air-borne particles from increasing the reflectivity of the internal surfaces around the dark space.

According to the invention apparatus for detecting fluid-borne particles comprises means projecting illumination along a first light path in said space for scattering from said particles, means detecting scattered light along a second light path in said space intersecting the first path, characterized in that respective paths are at least partly bounded by walls respectively entirely obstructed from each other so that light in the first path adjacent one wall is blocked from said second wall.

Further according to the invention the aforesaid space is enclosed by light-obstructive means, a substantial part of said means comprising a body of porous material transmissive of said fluid and particles.

For the purposes of illustration certain embodiments of the invention are shown in the accompanying drawings, in which:

FIG. 1 is a plan view of a smoke detector according to the invention, partly broken away;

FIG. 2 is a side elevation of the smoke detector;

FIG. 3 is a greatly enlarged view of porous material forming part of the smoke detector; and

FIG. 4 is a schematic view, similar to FIG. 1, showing a modified form of smoke detector.

As shown in FIGS. 1 and 2, a smoke detector comprises a solid, triangular block 1 of opaque material with a surface 2 facing into a dark chamber 3 surrounded by a semicircular housing 4 including a circular wall 6, an upper plane wall 7 and a lower plane wall 8. The tri- 3,497,303 Patented Feb. 24, 1970 angular block 1 has a first ball socket 9 in which a spherical lens 11 is held by a ring 12. A lamp fixture 13 attached to the block 1 holds a lamp 14 on the axis 15 of the ball socket 9 and lens 11. A second ball socket 16 contains a like spherical lens 17 on an axis 20. To the rear of the lens 17 a photocell 18 is secured in the ball socket 16 by an opaque insulating plate 19 attached to the block 1.

Extending from the ball sockets 9 and 16 through the block 1 to the surface 2 are tubular light passages 21 and 22, defined by interior walls of the block 1, whose axes are disposed at an angle to each other. Light from the lamp 14 is collected by the lens 11 and projected along a first path 23 on the axis 15 through the passage 21 into the dark chamber 3. The photocell 18 views along a second path 24 on the axis 20 through lens 17, light passage 22 and a zone 26 in which the light paths 23 and 24 intersect. Preferably the lamp' 14 is located as closely as possible to the lens 11 with three advantageous results. First, the dimensions of the detector are reduced. Secondly, the solid angle of radiation from the lamp to the lens aperture is increased providing optimum light utilization. And thirdly, by using a short focal length lens, an intense image of the lamp filament is imaged at a location 25 in the intersection zone 26. Thereby the most intense light is provided where light is scattered to the photocell 18, and the light in path 23 is weakest at the area 6a of the wall 6 where it might be reflected undesirably to the photocell.

Air-borne smoke particles are admitted to the dark chamber 3 through the walls 6, 7 and 8 of the housing 4, which walls are of porous material as will be more fully explained. Smoke particles entering the intersection zone 26 receive light from the lamp 14 and scatter the light, some of the light being scattered along the path 24 to the photocell 18 which thereby senses the presence of the smoke particles.

The major portion of the housing 4 surrounding the chamber 3 in which the smoke is to be detected is formed of a porous material P which is transmissive of air-borne smoke particles but substantially non-transmissive of outside light. As shown the housing walls 7 and 8 comprise opposite flat face portions of such material P in sheet form, and the connecting, generally semi-circular wall 6 of the same sheet material P, the three pieces being glued together, and engaging over the corner portions of the block 1 to which they may be fastened in any suitable way, such as by stapling.

The porous material P is preferably comprised of a skeletal three-dimensional network of light-blocking material which in the aggregate thickness of the sheet material is effective to exclude outside light from the chamber, but provides interconnected pores transmissive of airborne smoke particles through the sheet material. Such sheet material, in a range of porosities and thicknesses, is commercially available in the form known by the names of Scott Industrial Foam and Scott Filter Foam, an ester type polyurethane foam available from Scott Paper Company of Chester, Pa. Transrnissibility of air currents through this material is a function of the number of pores per linear inch appearing on the surface of the material, and the light-blocking property of the material is also a function of this number of pores per linear inch. The transmissibility of air currents varies inversely with the thickness of the material, and the light-blocking property varies directly with the thickness of the material up to a point where substantially total obscurity is reached.

As shown in the diagrammatic view, to greatly enlarged scale, of FIG. 3, the skeletal network of the material is porous and is three-dimensional and provides pores which occur at random, but interconnect so as to afford air passages through the material. In the very thin slice depicted in FIG. 3, the material may be seen to be light-transmissive. However, as thickness increases the light-blocking aggregate effect of the random disposed strand-like portions increases, so that at a sufiicient thickness total obscurity is obtained. It is of course impossible to illustrate in this type of diagram the fact that even though nearly total light obscurity is obtained, air transmissibility remains.

This material, when it is of inch thickness and averages approximately 90 pores per linear inch, provides adequate transmissibility for the air currents and almost totally obscures light which is aimed directly at the surface of the material. It will completely obscure light coming from a somewhat oblique direction which would require a longer path of travel of the light through the material. This material of 80 pores per inch can best be used in conjunction with an opaque patch or mask 6a on the inner or outer surface in the region directly opposite to the opening which leads to the photocell. An alternative to masking this portion of the wall of the chamber is to simply attach there an additional thickness of the porous sheet material, with the effect of causing substantially total blocking of outside light at that place.

Such material having a thickness of 7 inch'and with an average of approximately 40 pores per linear inch will satisfactorily admit the smoke-bearing air currents but exclude the outside light or a material 7 inch thick with 80 pores per linear inch is satisfactory. For installations which are to operate in known drafts of substantial amounts, usually ducts or conduits, material inch thick and averaging approximately 100 pores per linear inch may be used.

Other materials, such for example as fiber glass, having the ability to exclude light but to transmit the smokebearing air currents according to the same principle may be used, but the aforesaid polyurethane foam material is preferred because its strand-like portions are part of a continuum, all contribute strength to the material, and have no tendency to become dislodged or irregular in density. As indicated above, the interior surface of the chamber should be optically black or nearly so, to prevent reflection of light from the beam from the incandescent lamp. The inner surface 2 of the block 1, facing the chamber, and also the walls of passages 21 and 22 may be painted optically black, and the inner surface of the housing maybe sprayed or suitably dyed optically black or nearly so. This blackness of the inner face and the fibres in the thickness of the housing 6 is particularly important in the region 611 which is struck by the light beam from lamp 14, which region 6a is blackened not only on its inner surface, but also over the surface of the fibres beyond this surface. But it is good practice to blacken the whole interior surface of the sheet material. In addition the skeletal structure acts to absorb light by admitting a portion of the light to various depths, from which depths reflected light will be absorbed by the sidewalls and trapped by the back walls of the skeletal material. And in practice all the interior and exterior surfaces of the fibres of the housing 6 may be blackened to render them lightabsorbing and opaque. And the area 6b viewed bythe photocell 18 may be completely masked from outside light by an opaque barrier 6b on the inner surface of the housing 6 or within the interstices of the housing, or on its outside surface.

Dust in the nature of lint will be excluded by the finely porous material, and hence will not settle on the initially optically black inner surfaces of the chamber.

Some dust may be expected to settle on the outside surface of the porous wall, but this may be expected to occur preferentially at some place on the surface, so that by making this surface relatively large, relatively unobstructed areas will remain for the transmission of air currents in and out of the chamber.

As compared to typical smoke detector devices the device of the present invention has the advantage of admitting a comparable amount of smoke through many small orifices that are distributed over a large surface area as opposed to one or more large orifices in an otherwise impervious enclosure. This feature provides for the admittance of smoke through any portion of the skeletal material which forms the major portion of the enclosure of the sensing chamber and thus will admit smoke from a Wide range of directions. A further advantage of the irregular skeletal construction is its ability to trap within its body foreign particles of a size smaller than the individual pores of the structure. Incidental to its other function of excluding outside light the skeletal structure also excludes insects which if admitted could become a reflecting source so as to cause false detection.

A further advantages of the present invention derives from the particular disposition of the walls forming the light passages 21 and 22. The interior walls of the block 1 defining the light passages are inclined at right angles to each other and at 45 to the surface 2 of the block 1. The surface 2 thus lies in a plane which truncates both passages. The walls bounding the passages are obstructed from each other by an intermediate portion of the block 1 terminating in a part 2a of the truncating surface 2. Therefore light reflected from the walls of the light passage 21 cannot directly strike the walls of the photocell passage 22. Further, the edges 2b formed by the junction of the partial surface 2a and the passage 21 and 22 are spaced by the partial surface 2a so that light diffracted around one edge 2b cannot enter the passage bounding the other edge 2 b, and dust collecting on either edge cannot scatter light into the photocell passage 22.

A similar obstruction of light can be obtained as shown in FIG. 4, wherein a circular, porous housing 4' is entered by two tubes 21' and 22 respectively enclosing an illuminating passage and a photocell passage. The inner ends of the tubes are truncated by a common plane as with the passages 21 and 22 of FIG. 1.

It can be seen that the adavantages of a porous housing and truncated light passages are obtained by the forms of FIG. 1 or 4 and variations thereon. Thus it should be understood that the present invention includes all modifications and equivalents falling within the scope of the appended claims.

We claim: 1. Apparatus for detecting fluid-borne particles in a space comprising means projecting illumination along a first light path in said space for scattering from said particles,

means detecting scattered light along a second light path in said space intersecting the first light path,

means supporting said means projecting illumination and said means detecting scattered light,

substantially light-obstructive means enclosing said space so as to exclude ambient light from said space, a substantial part of said light-obstructive means comprising a body of porous material transmissive of said fluid and particles, and said porous material comprising a skeletal three-dimensional network of light-blocking strand-like portions effective in. the aggregate to exclude outside light from the chamber but forming interconnected pores transmissive of air-borne smoke particles through the material into the space.

2. Apparatus according to claim 1 wherein the inner surface of said body on the area where it intersects with said first light path is highly light-absorptive.

3. Apparatus according to claim 1 wherein said porous material has pores which average in number from approximately 40 to approximately per linear inch.

4. Apparatus according to claim 1 wherein the inner portion of said porous material, in the area where it intersects With the first light path, is substantially optically black.

5. Apparatus according to claim 1 wherein the inner portion of said porous material, in the area where it intersects with the second light path, is substantially optically black.

6. Apparatus according to claim 1 wherein part of said light-obstructive means comprises a unitary body of opaque material connected to said body of porous material and forming walls about said light paths.

7. Apparatus for detection of air-borne smoke particles comprising:

an opaque body 1 having a first light passage 21 and a relatively inclined second light passage 22 formed therein and opening on a plane surface 2 of the body,

means 11, 14 for projecting illumination along a first light path 23 through the first passage into a space 3 bounded by said surface 2, means 18 for detecting light from smoke particles in said first path within said space reflected along a second path 24 through said second passage to said detecting means, said first and second paths intersecting in said space and being truncated by said plane surface 2,

wall means 6, 7, 8 connected to said opaque body and forming therewith an enclosure for said space, said wall means comprising a porous material P formed of a skeletal three-dimensional network of light- JEWELL H.

WARREN A. SKLAR, Assistant Examiner blocking strand-like portions effective in the aggregate to exclude outside light from the chamber but forming interconnected pores transmissive of air-borne smoke particles through the material into the space, the inner Wall of said material being substantially optically black at least in said first path 15, and

said opaque body comprising a fiat surface 2a connecting said two passages in the plane surface truncating said passages.

References Cited UNITED STATES PATENTS PEDERSEN, Primary Examiner US. Cl. X.R. 

